In the medical field, a common procedure performed by emergency medical technicians, phlebotimists, nurses, doctors, and other medical field personnel is locating veins and inserting intravenous (IV) tubes (e.g., catheters) and blood drawing needles therein. One problem that is often encountered when administrating an IV or drawing blood is that it is often difficult for the medical person to locate a vein. This problem is particularly pronounced with obese or pediatric patients, or when conditions are less than favorable, for example on a battlefield or at an accident scene. Various methods and devices have been devised to help a user to locate a vein that would be otherwise difficult to locate.
One popular method for locating a vein is the Doppler sonar method. In this method, a transmitter (or transceiver) provides a high frequency sound signal that is transmitted through the surface of a patient's skin in an effort to locate a vein. Although sound will be scattered by all tissues, for the blood flowing through a vein, the signal will be Doppler shifted due to the motion of the blood. If the velocity of the blood is v, the received signal (received at the transducer of a transceiver or receiver) is shifted in frequency as provided in Equation 1 below:
                              Δ          ⁢                                          ⁢          f                =                  2          ⁢          f          ⁢                      v            c                    ⁢          cos          ⁢                                          ⁢          θ                                    Eq        .                                  ⁢        1            where f is the transmitted frequency (e.g., 10 Mega-Hertz (MHz)), c is the speed of sound in the tissue (approximately 1500 meters per second (m/sec)) and θ is the angle between the flow velocity direction and the sound beam axis. For a typical vein, the flow velocity is of the order of 10 cm/sec. If θ is, for example, 30°, the frequency shift will be 667 Hz. Such shifts can be detected by “beating” the received signal against the transmitted signal. The presence of an audio frequency beat signal in the output (in this case at 667 Hz) indicates a Doppler shift, and hence that the beam is intersecting a blood vessel such as an artery or vein. Since arterial flow is away from the heart and venous flow is towards the heart, the distinction between arteries and veins is made by determining whether the Doppler shift is positive or negative. Both the size of the Doppler shift and the strength of the Doppler shifted signals are helpful in selecting the most suitable vein since a larger vein has higher flow velocities in addition to being a better scatterer.
Although the technology used to locate a vein has improved, the methods used to insert a needle quickly and accurately using a hand-held device and at a moderate cost could benefit from further development.